Impact of Chiral Symmetry Breaking on Spin-Texture and Lone Pair Expression in Chiral Crystals of Hybrid Antimony and Bismuth Halides
Annalise E. Maughan, Gabrielle Koknat, Peter C. Sercel, Manoj K. Jana, Roman Brunecky, David B. Mitzi, Joseph J. Berry, Volker Blüm, Matthew C. Beard
Abstract
Chiral organic building blocks can be incorporated into hybrid organic–inorganic metal-halide crystalline semiconductors so as to control and impact the interconversion between light, charge, and spin. Here, we report a series of hybrid antimony and bismuth halide materials of the general formula MBA 4 B 2 X 10 (B = Sb 3+, Bi 3+; X = Br –, I – ) and study how chiral symmetry breaking imposed by the templating organic chiral methylbenzylammonium (MBA) cations induces symmetry breaking within the inorganic sublattice and leads to a unique spin-texture. The chiral MBA cations introduce two structural modifications to the metal-halide sublattice that consists of isolated edge-sharing dimers of B 2 X 10 octahedra: (1) the dimers have a chiral spatial arrangement with respect to one another and (2) there is an asymmetric distortion of the two subunits within the individual dimers with a higher distortion caused by strong stereochemical activity of the Sb 5s 2 lone pair electrons. The structural distortions and chiral arrangement result in circular dichroism (CD) at the band edge of the inorganic framework with dissymmetry factors in the range of 10 –4 . Chiral spin-splitting of the inorganic states, caused by breaking of the inversion symmetry and large spin–orbit coupling, is studied via density functional theory (DFT), and a multiband effective mass theory was developed that links the DFT-derived spin-splitting of helical character (i.e., spin expectation value not perpendicular to the crystal momentum) to the observed CD. We also find broad red photoluminescence from the MBA 4 Sb 2 Br 10 compounds, which we attribute to self-trapped excitonic emission driven by the large distortion due to the lone pair expression.